In the prior art, liquid silicone rubber compositions of the addition reaction curing type are regarded as being useful for the manufacture of medical parts including valve gaskets and nipples because of their advantages in moldability and toxicity as compared with silicone rubber compositions of the peroxide curing type known as millable type. Since they are less satisfactory in flexural resistance, extension resistance, and other fatigue resistance properties requisite for such applications, there is a need for further improvements from practical aspects.
A variety of techniques are known for improving the fatigue resistance of silicone rubber compositions, particularly millable silicone rubber compositions. For example, it is useful to blend a linear organohydrogenpolysiloxane having locally concentrated vinyl groups in raw rubber compositions as disclosed in Japanese Patent Application Kokai No. 197457/1987, to add an organic phosphate salt or titanate coupling agent as disclosed in Japanese Patent Application Kokai No. 41263/1987, and to use a blending co-agent in the form of a copolymer of dimethylsiloxane and 3,3,3-trifluoropropylmethylsiloxane as disclosed in Japanese Patent Application Kokai No. 150350/1988.
If such techniques are applied to liquid silicone rubber compositions of the addition reaction curing type, curing inhibition and a considerable loss of flow occur. The techniques are ineffective for these compositions, which are thus difficult to improve fatigue resistance properties.
U.S. Pat. No. 4,360,610 discloses that a silicone rubber having improved tensile strength is obtained by adding a hydroxylated vinyl group-containing organo silicon compound in two increments to a mixture of a polydiorganosiloxane gum and an untreated reinforcing filler and curing the resulting compound with an organic peroxide. However, this silicone rubber composition relies on an organic peroxide curing system. The fatigue resistance of the composition is described nowhere.
There is a need for developing a technique for improving the fatigue resistance of liquid silicone rubber compositions of the addition reaction curing type.